MySQL Error code 1452 occurs but values and tables do exist - mysql

Question

I cannot for the life of me figure out where did I mess up. I Have the following tables, made using this SQL code:
CREATE TABLE IF NOT EXISTS Users(
username VARCHAR(50) NOT NULL PRIMARY KEY,
pwd TEXT NOT NULL,
first_name VARCHAR(25) NOT NULL,
last_name VARCHAR(25) NOT NULL,
user_type VARCHAR(25) NOT NULL,
user_status VARCHAR(10) NOT NULL,
email VARCHAR(255) NOT NULL
);
CREATE UNIQUE INDEX Users_Index
ON Users (username);
CREATE TABLE IF NOT EXISTS Clients(
client_id VARCHAR(10) PRIMARY KEY,
client_name VARCHAR(255) NOT NULL UNIQUE,
first_name VARCHAR(25) NOT NULL,
last_name VARCHAR(25) NOT NULL,
middle_name VARCHAR(25),
related_to VARCHAR(10),
relation_type VARCHAR(25),
ssn_ein VARCHAR(11) NOT NULL UNIQUE,
date_of_birth DATE NOT NULL,
manager VARCHAR(50) NOT NULL,
entity_type VARCHAR(50) NOT NULL,
client_status VARCHAR(15) NOT NULL,
address VARCHAR(255) NOT NULL,
city VARCHAR(255) NOT NULL,
state VARCHAR(2) NOT NULL,
zip INT NOT NULL,
email VARCHAR(255),
phone VARCHAR(255) NOT NULL,
notes TEXT,
FOREIGN KEY(manager) REFERENCES Users(username)
);
CREATE UNIQUE INDEX username_index
ON Clients(manager);
I tried to insert data into the Clients table but it gave me error 1452, which is for when a constraint check has failed. Thing is, I already have data in the Users table, I TRIPLE checked for typos, I added the indexes to both tables to no avail, I checked that the reference was pointing to the right table, I checked to see that the engines were both InnoDB as well as the collations being exact, and both username and manager columns are the same exact datatype. I have checked the MySQL documentation and followed all requisites to the best of my knowledge. Does anyone know what could be the problem?
Here is the sample data:
INSERT INTO Clients VALUES (
client_id = "123456",
client_name = "John Doe",
first_name = "John",
last_name = "Doe",
middle_name = "Michael",
related_to = NULL,
relation_type = NULL,
ssn_ein = "123-456-7890",
date_of_birth = "1990-01-01",
manager = "OzzyTheGiant",
entity_type = "Individual",
client_status = "Active",
address = "123 Main St.",
city = "Anytown",
state = "XX",
zip = 78550,
email = "john.doe#example.com",
phone = "123-555-7890",
notes = "self-employed"
);

Answer 1

That's cause the row/record you are trying to insert into Clients table doesn't exists in Users table and thus the error. Since the record doesn't exists in parent table itself, how can you expect to insert in child table? that's not possible.
Also, your INSERT query syntax wrong. Your schema creation script should look like below. See a demo fiddle here: Demo Here
CREATE TABLE IF NOT EXISTS Users(
username VARCHAR(50) NOT NULL PRIMARY KEY,
pwd TEXT NOT NULL,
first_name VARCHAR(25) NOT NULL,
last_name VARCHAR(25) NOT NULL,
user_type VARCHAR(25) NOT NULL,
user_status VARCHAR(10) NOT NULL,
email VARCHAR(255) NOT NULL
);
CREATE UNIQUE INDEX Users_Index
ON Users (username);
CREATE TABLE IF NOT EXISTS Clients(
client_id VARCHAR(10) PRIMARY KEY,
client_name VARCHAR(255) NOT NULL UNIQUE,
first_name VARCHAR(25) NOT NULL,
last_name VARCHAR(25) NOT NULL,
middle_name VARCHAR(25),
related_to VARCHAR(10),
relation_type VARCHAR(25),
ssn_ein VARCHAR(11) NOT NULL UNIQUE,
date_of_birth DATE NOT NULL,
manager VARCHAR(50) NOT NULL,
entity_type VARCHAR(50) NOT NULL,
client_status VARCHAR(15) NOT NULL,
address VARCHAR(255) NOT NULL,
city VARCHAR(255) NOT NULL,
state VARCHAR(2) NOT NULL,
zip INT NOT NULL,
email VARCHAR(255),
phone VARCHAR(255) NOT NULL,
notes TEXT,
FOREIGN KEY(manager) REFERENCES Users(username)
);
CREATE UNIQUE INDEX username_index
ON Clients(manager);
INSERT INTO `Users` VALUES('OzzyTheGiant', 'scxcxx','test1','test2','test','test','test');
INSERT INTO Clients VALUES (
'123456',
'John Doe',
'John',
'Doe',
'Michael',
NULL,
NULL,
'123-456-78',
'1990-01-01',
'OzzyTheGiant',
'Individual',
'Active',
'123 Main St.',
'Anytown',
'XX',
78550,
'john.doe#example.com',
'123-555-7890',
'self-employed'
);